P0556: Brake Booster Pressure Sensor Circuit Range or Performance

What it means

Same as P0555 with the signal in plausible range yet not matching the engine model - drift, partial blockage of the reference port.

At a glance

• Usually lights the check-engine light

How it works

The brake-booster pressure sensor reports pressure in the booster to the control module. The module compares the signal with the pressure expected from engine conditions and sets P0556 when the reading stays within the electrical range but does not track the expected pressure, possibly because the sensor has drifted or its pressure port is restricted.

Common causes

Most likely first.

1. Brake booster pressure sensor drift or deterioration
2. Blocked or restricted sensor pressure port
3. Poor electrical connection at the sensor
4. Damaged sensor wiring

Symptoms

• Check-engine light on
• Stop-start or brake-assist features may be disabled

How to diagnose it

1. Record freeze-frame data and compare brake-booster pressure with engine conditions in live data to see when the reading stops tracking expectation.
2. Check the sensor connector and wiring for loose pins, corrosion, damage, or poor contact.
3. Inspect the sensor pressure port and its passage for dirt or restriction.
4. Check the sensor reference, ground, and signal circuit against the manufacturer's specification.
5. If the circuit and port are clear, test the brake-booster pressure sensor against the manufacturer's specification.

Common fixes

• Clean or repair a restricted sensor pressure port.
• Repair a loose, corroded, or damaged sensor connector.
• Repair damaged sensor wiring.
• Replace a brake-booster pressure sensor that fails testing.

Can I keep driving?

If braking feels normal and no brake-assist warning is present, drive cautiously and arrange diagnosis soon; stop-start or brake-assist features may be unavailable. Stop if the pedal becomes hard or braking effort increases.